<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue.js"></script>
</head>
<body>
  <div id="app">
      <h3>{{username}}</h3>
      <h3>{{10+20}}</h3>
      <h4>{{new Date().toLocaleString()}}</h4>
  </div>
<script>
    new Vue({
        el: "#app",
        data: {
            username: "Jack",

        }
    })
</script>
</body>
</html>